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On 6/1/2011 8:41 AM, Larry Crutcher, Starlight Solar Power Systems
wrote:
<blockquote
cite="mid:EF24CA1F-9FA6-4A3C-AC39-DA7EB9525816@starlightsolar.com"
type="cite">Hi boB,
<div><br>
</div>
<div>We know that current will flow through the lowest resistive
path. In a perfect world, the resistance from end to end of a
battery string will be identical. Real world conditions show
that one or more cells have a slightly higher internal
resistance. Add to that the imperfect terminal crimp, improper
torque at battery terminal, and perhaps terminal corrosion and
we now have paths of differing resistance. The parallel strings
have an unequal distribution of current and ergo undercharged
cells. These cells retain a small amount of amorphous lead
sulphate that, without correction, will begin to crystalize. We
all know what will happen over time. </div>
<div><br>
</div>
</blockquote>
<br>
The issue of maintenance is a not brainer of course... i.e. the
need to add water...<br>
<br>
Imperfect crimps would be a problem with a single string of
batteries too. If the resistance<br>
is higher in that cell in the one string, then yes, that one string
will not be working nearly as<br>
hard as the other strings. Bad workmanship is not an excuse for not
using multiple strings<br>
as far as I know.<br>
<br>
<br>
If one string is discharged more than another parallel string, then
its finishing (ending) Amps<br>
will will certainly happen later than a string that wasn't
discharged as deep, but if using a<br>
timed Absorb cycle, why does this matter ?? They're all receiving
the same voltage and<br>
should be treated the same in that case. Right ?? At least for
timed charging cycles.<br>
<br>
As Darryl pointed out, AGMs like to get hot when unbalanced and this
is definitely what I<br>
have seen. But that again can happen for a single string rather
than multiple strings.<br>
<br>
One thing that hasn't been mentioned, unless I missed it, is the
possibility that a shorted<br>
cell in one string can drag down the other strings it is in parallel
with. The more strings<br>
in parallel, statistically, there would be more of a chance for that
to happen.<br>
<br>
That would be more of a self discharge issue I would think which
isn't good.<br>
boB<br>
<br>
<br>
<br>
<br>
<blockquote
cite="mid:EF24CA1F-9FA6-4A3C-AC39-DA7EB9525816@starlightsolar.com"
type="cite">
<div>Granted, this can be prevented by the proper knowledge and
attention but I see far too many failures from neglect and/or
lack of understanding. My experience comes from 10 years of
working with hundreds of small 12 volt systems with 2 to 6
parallel strings of batteries. In our shop we often see 2 to 3
year old battery banks that are damaged and I believe this is a
contributing factor. It is often too late by the time the
problem is discovered. So, my opinion is to reduce this
possibility by decreasing the parallel paths whenever possible.</div>
<div><br>
</div>
<div>More strings of a given size battery means less stress per
string. But, if you decrease strings you are normally increasing
cell size to maintain the capacity so this should not be an
issue.</div>
<div><br>
</div>
<div>Battery balancing is a great idea. I would like to see an
active system for shunting current on a string to keep an equal
current across all strings. I know this is done in the EV
business.</div>
<div><br>
<div>
<span class="Apple-style-span" style="border-collapse:
separate; color: rgb(0, 0, 0); font-family: Helvetica;
font-style: normal; font-variant: normal; font-weight:
normal; letter-spacing: normal; line-height: normal;
orphans: 2; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
font-size: medium;">
<div>Larry Crutcher</div>
<div>Starlight Solar Power Systems</div>
<div>11871 S Fortuna Road, #210</div>
<div>Yuma, AZ 85367</div>
<div><br>
</div>
<div><a moz-do-not-send="true"
href="mailto:larry@starlightsolar.com">larry@starlightsolar.com</a></div>
<div>(928) 342-9103</div>
<div><br>
</div>
</span><br class="Apple-interchange-newline">
</div>
<br>
<div>
<div>On May 31, 2011, at 11:16 PM, boB Gudgel wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
<div bgcolor="#ffffff" text="#000000"> <br>
<br>
OK, I've been hearing for years now that one battery
string is best and maybe two but no more should be used.<br>
<br>
What is the reason for this fear ??? What is the
experience with multiple strings in parallel ??<br>
<br>
If it is because of current sharing of the strings, that
shouldn't be an insurmountable problem if the wiring is
done right.<br>
<br>
If it has to do with the possibility of fire, which Darryl
mentioned, seems that series string fuses could mitigate
the possibility of that issue, if because of shorted
cells, just like PV combiners.<br>
<br>
Or maybe it has to do with the trouble of getting into the
middle of a pile of batteries ? Use battery boxes ?<br>
<br>
The strings are in parallel and so they all get the same
voltage, theoretically anyway if the connections are good.<br>
<br>
I would think that the more strings, the LESS stress on
individual batteries, not more. The current in each
battery should be less than it would be I would think.<br>
<br>
So what is the real experience ?? <br>
<br>
Is a battery balancer the answer ??<br>
<br>
Thanks,<br>
boB<font class="Apple-style-span" color="#000000"><font
class="Apple-style-span" color="#144fae"><br>
</font></font></div>
</blockquote>
</div>
</div>
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